Good catch, though the issue is more subtle than that. Your solution lets the type ShouldntSync below pass:

class ShouldntSync : ISyncSimple
{
    SomeType test;
}

class SomeType : ISyncSimple
{
    NotSyncable test;
}

The calls to CanHandle on ISyncSimples can follow arbitrary graphs and the problem here is of cycle prevention. I'll fix it in a future update.

Where do you need to use generic ISyncSimple fields? The simplest fix for this case is to replace
All(f => CanHandle(f.FieldType)) with
All(f => f.FieldType == typeof(ISyncSimple) || CanHandle(f.FieldType)).

Where do you need to use generic ISyncSimple fields?

My plan was to slightly modify the styling station patch so other mods could hook into it. Basically, I wanted to make a compat patch for Humanoid Alien Races (currently causes exceptions and breaks the dialog with MP due to dummy pawn not having all required HAR data), but I can't without slight changes to MP itself. So I've wanted to (among other things) add public List<ISyncSimple> additionalData to StylingData so mods could sync extra data from the dialog.

The simplest fix for this case is to replace
All(f => CanHandle(f.FieldType)) with
All(f => f.FieldType == typeof(ISyncSimple) || CanHandle(f.FieldType)).

I originally did something similar, but then I just kept finding more and more edge cases. For example, this won't handle the following:

// This is the one I rely on for the changes I planned to make
class TestSync : ISyncSimple
{
    List<ISyncSimple> test;
}

class Test : ISyncSimple
{
    Test test;
}

interface IExtendedSyncSimple : ISyncSimple
{
}

class Test : IExtendedSyncSimple
{
    IExtendedSyncSimple test;
}

// This example is where I finally gave up and made a simple solution
class TestFirst : ISyncSimple
{
    TestSecond test;
}

class TestSecond : ISyncSimple
{
    TestFirst test;
}

I think adding

if (type == typeof(ISyncSimple))
    return true;

before

if (typeof(ISyncSimple).IsAssignableFrom(type))

will suffice for now. Can you make this change?

I'll try to make that change tomorrow, it's a bit late right now.

I've made the changes. This should handle the most common issues, but could fail in more complex (and rare) situations.

Included in #431